xe2x80x9cThis application is a national phase of PCT/FR00/00918 which was filed on Apr. 11, 2000, and International Application No. 99/04726, which was filed on Apr. 15, 1999, and was not published in English.xe2x80x9d
The purpose of this invention is a process for the determination of the permeability of a magnetic material by disturbance of a coaxial line. Its applications include characterization of materials, and particularly ferromagnetic materials.
A network analyzer can be used to measure the reflection and transmission of a hyper frequency electromagnetic wave on and through a sample placed in a wave-guide. If this wave guide is a coaxial line, for example according to the APC7 standard that can carry a frequency band of up to 18 GHz, the permittivity xcex5 and the permeability xcexc of the sample can be deduced from this measurement using the Nicholson and Weir equations [1, 2]. These equations are only applicable to a solid homogenous isotropic sample, and therefore many materials are excluded.
If the sample to be characterized is a very good conductor, the wave reaching the sample is entirely reflected, the transmission coefficient is zero and the reflected signal contains no information other than the highly metallic nature of the sample. Furthermore, thin metallic layers and particularly thin ferromagnetic layers cannot be directly characterized using this method.
Variants have been developed using other types of guides, but they cannot be used to measure the permeability of thin ferromagnetic layers for the same reasons.
Furthermore, methods using rectangular guides or micro-ribbon lines have been developed for non-metallic anisotropic samples. These samples only partially fill the guide, since sample dimensions are unacceptable (between 350-500 MHz, the dimensions are 53.4 by 27.9 cm). But these methods lead to very complex numeric calculations [3] compared with the solution of the Nicholson and Weir equations. Furthermore, the frequency band of each guide is narrow so that the guide has to be changed several times, and therefore the sample dimensions have to be changed to cover a wide frequency band.
The turn disturbance method is a simple method for characterization of thin ferromagnetic layers. It may be considered as being a short circuited micro-ribbon line, or it can be modeled as a square turn in which the inductance is disturbed by the presence of a magnetic sample. This method suffers from two major limitations:
1xc2x0) the turn must be calibrated using the measurement of a known sample; however, there is no standard hyper frequency permeability, and no simple means of relation to a primary standard;
2xc2x0) Apart from the limitation of its frequency band, a demagnetizing field is introduced by the parallelepiped shape of the sample [4]; since the dimension of the sample is limited in the direction parallel to the hyper-frequency magnetic excitation, poles are created at the ends which modifies the permeability of the sample compared with a larger sample. This effect gets weaker as the ratio between the dimension parallel to the excitation and the thickness increases.
The size limitation of the turns is related to their high frequency nature, and therefore it is not possible to increase the dimensions without reducing the accessible frequency band.
In the case of materials in the form of wires, it is known that the magnetic properties of wires change very considerably below a certain length. Therefore, it is very penalizing to be obliged to restrict measurements to short samples, as is the case for measurement in turns.
Document FR-A-2 699 683 describes a process for determination of the intrinsic magnetic permeability of elongated ferromagnetic elements in which a wound torus is made with ferromagnetic elements, this torus is placed in a coaxial line, the properties of the torus are measured and the permeability of the ferromagnetic elements is deduced. This method has the advantage that it does not require a permeability standard. It also covers a wide frequency band (from 50 MHz to 18 GHz). Finally, the magnetic sample is long in the direction of the hyper frequency magnetic excitation and there is no risk of creating demagnetizing effects.
However, this method has disadvantages. Firstly, it needs much greater quantities of material than the measurement in turns: several tens of mm3, compared with a tenth of a cubic millimeter or less for turn techniques. Methods of making thin layers and wires are rarely compatible with the production of a few tens of cubic millimeters of material. Even if is possible to acquire the right quantity of material, it is still difficult to be sure about the uniformity of the properties of the rest of the material used to make the sample.
Secondly, the fact of winding the flexible material starting from the inside diameter of the coaxial lime results in a very small radius of curvature of the material, of the order of 1.5 mm for an APC7 line. However, it is known that the risk of observing magnetostrictive effects that affect material properties increases as the stress increases. The fact that the curvature varies considerably between the inside diameter and the outside diameter of the line makes any attempt at correcting or taking account of this effect illusory.
Finally, the last disadvantage of the measurement of a wound torus is that preparation of the sample is long and difficult. Several meters of ferromagnetic ribbon or several hundreds of meters of wire has to be prepared. In practice, the need for precise dimensions for a sample makes machining operations essential and glue has to be used to hold the many ribbon turns, that introduces badly controlled stresses. Removal of magnetic material created during machining causes an inaccuracy about the content of magnetic material, which affects the permeability of the ferromagnetic constituent.
The purpose of this invention is to overcome these disadvantages.
The invention makes use of the idea of a measurement in a coaxial line disturbed by the sample, as described in FR-A62 699 683, but with the fractional volume of the sample being much smaller. In this invention, this fraction (which is the ratio between the volume of the sample and the volume of the part of the coaxial line on which the sample is located) is less than 1%. This fraction is much greater in prior art (several tens of a percent). In the examples that will be described later, the fractional volume is as lower as 0.8% or even 0.06%. These very low values make it necessary to find a new solution to the problem of determining the permeability (the formulas given in the patent mentioned are no longer valid). Furthermore, precautions have to be taken to prevent or reduce the demagnetizing fields when making the sample.
More precisely, the purpose of the invention is a process for determination of the permeability of a magnetic material by disturbance of a hyper frequency coaxial line, in which a sample of the said material is formed, this sample is placed in a hyper frequency coaxial line, the reflection and/or transmission of a hyper frequency electromagnetic wave through this coaxial line is measured and the result of this measurement is used to deduce the magnetic permeability of the material, this process being characterized in that the fractional volume of the magnetic material in the sample is less than 1% of the volume of the disturbed coaxial line.
The sample can be created by winding a wire or a ribbon on a toric support, for example an insulating support, or a thin layer can be deposited on such a support. The ribbon can be obtained by depositing a thin layer of magnetic material on a flexible substrate and cutting a ribbon in this substrate along a determined direction. Therefore the permeability measurement will be related to this direction. The direction can be changed to explore the permeability in several directions.
One end of the wire or ribbon can be fixed on the toric support, tension can be applied to this wire or this ribbon, and then the second end of the wire or ribbon can be fixed on the support. The permeability under stress is thus measured, which is different from the stress free permeability due to magnetostrictive effects.
The sample can also be formed directly on conductors inside or outside the coaxial line.
The sample may be in the form of a complete torus or simply a toric sector.
The magnetic material for which the permeability is to be measured may be arbitrary, and in particular it may be ferromagnetic.
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FIG. 1 shows the variations of the magnetic permeability of a wire as a function of the frequency, for measurements by turn disturbance and by coaxial line disturbance.
FIG. 2 shows variations of the magnetic permeability of a thin ferromagnetic layer as a function of the frequency for measurements by turn disturbance, by coaxial line disturbance and by wound torus.